void reverse(SqList *&L)
{
    int c;
    for(int i=0;i<L->length/2;i++)
    {
        c=L->data[i];
        L->data[i]=L->data[L->length-i-1];
        L->data[L->length-i-1]=c;
    }
    L->length=c+1;
}
